Field of the Invention
This invention relates to target tracking and laser beam steering to illuminate the tracked targets for designation, range finding or active imaging, and more particularly to tracking and illumination of multiple targets per frame, one target at a time in a field-of-view (FOV).
Description of the Related Art
Laser beam steering is used to perform functions such as Designation, Range Finding and Active Imaging. Typically, a laser is configured to transmit a laser beam, typically pulsed, along a fixed transmission path (“along boresight”). The laser may be steered manually in a hand-held unit or automatically on a gimbal mounted system to point boresight at the target.
Laser Designation transmits an encoded pulsed laser beam at a wavelength of 1,064 nm to designate a target. The pulsed laser beam has a pulse repetition frequency (PRF) in which a defined pattern of pulses forms a designation code. Laser Designation of targets is used during acquisition, tracking and terminal guidance of guided munitions with a sensor commonly known as a semi-active laser (SAL) sensor.
Laser range finders transmit laser beams at a remote target to determine the distance or range to the remote target. Laser range finders generally operate on the “time of flight” principle by measuring the time taken for a laser pulse to travel to the target and be reflected back to the range finder. With the speed of the laser light being a known value, and with an accurate measurement of the time taken for the laser light to travel to the target and back to the range finder, the range finder is able to calculate the distance from the range finder to the target. Other techniques such as continuous wave (CW) or frequency modulated (FM) modulated CW may be used to determine range. An “eye-safe” wavelength of 1,550 nm is typical although 1,064 nm or other wavelengths may be used as well.
Active imaging detects laser energy reflected by elements within a scene to form an image of the scene. The active image of a portion of the scene may augment a passive image of the entire scene. Active imaging provides a measurably higher signal-to-noise ratio (SNR) than passive imaging, which can be useful for target detection, acquisition, classification or aimpoint selection.
U.S. Pat. No. 8,400,619 entitled “Systems and methods for automatic target tracking and beam steering” employs an image capturing system for acquiring a series ages in real time of a distant area containing a remote target, and a processing system for processing the acquired images to identify the target and follow its position across the series of images. An automatic beam steering system and method operate in conjunction with a laser source for emitting a laser beam to be transmitted in the form of a transmitted laser beam extending along a steerable beam transmission axis to the remote target. The beam steering system is controlled by the processing system to steer the beam transmission axis to be aimed at the target being tracked by the target tracking system, so that the transmitted laser beam will be transmitted at the appropriate angle and in the appropriate direction to be aimed at the tracked target “The beam steering system may accomplish steering of the beam transmission axis by decentering or adjusting the position of one or more components of an optical system, . . . ” (Col. 7, lines 6-14). This approach allows for small steering deviations off of boresight to designate a single tracked target.
The image processing system 34 controls the beam steering system 11 to compensate for positional changes of the target 26 by continuously adjusting the position of the laser source 15 as needed for the angle and direction of the beam transmission axis 25 to be aimed at the target 26 being tracked via the target tracking software 68. “Positional changes of the target may result from extraneous movement of the operator and/or movement of the target as discussed hereinabove.” (Col 14, lines 1-6). “The target tracking and beam steering processes are performed very rapidly, with images typically being processed within the time it takes for the next frame to be received, such that the target tracking system 10 will normally “lock” on the target and the range finder 12 will be ready for range acquisition very quickly after the operator has appropriately directed the forward or pointing end of the transmission system 14 toward the target 26. Once the target 26 is “locked”, activation of the range finder 12 to emit the laser beam 23 from the laser source 15 will result in the transmitted laser beam 24 being transmitted accurately to the target 26.” Col 14, lines 44-54. In lay terms, the operator points the weapon at the target, the image capture system determines a small correction to point the laser precisely at the target, the beam steering system mechanically moves the optical component to make the correction and once “locked”, the operator pulls the trigger to transmit the laser beam towards the single target. This method simply corrects the aimpoint for a single target.
Another class of problems involves tracking and illuminating multiple targets within a field-of-view (FOV) about boresight. An image capture system generates a list of tracked targets and angles-to-targets at the frame rate of the imaging system. One approach is to mechanically steer a laser spot-beam to illuminate different targets. Mechanical steering has size, weight, power and cost (SWaP-C) limitations that limit its effectiveness, especially for small platforms. Speed constraints limit the ability to illuminate multiple targets per frame within a FOV. Another approach is to non-mechanically steer a laser spot beam using optical phased arrays in combination with polarization gratings. This approach has a lower SWaP-C than mechanical beam steering but has a limited ability to illuminate multiple targets within a FOV.
The current state-of-the-art is to use a video camera and tracking card to generate the list of tracked targets and corresponding angles-to-targets, flood illuminate the FOV and simultaneously detect the reflected laser energy off of all of the targets in the FOV with an imaging detector. Flood illumination provides an active image of all of the targets in the FOV. This image may be correlated to the tracked targets and processed to compute the range to each of the targets. The SWaP-C of the laser to flood illuminate a FOV and the complexity of the processing to extract the range information and correlate it to the tracked targets is burdensome.